1. Field of the Invention
The present invention relates generally to thermoforming of sandwich panels comprised of fiber reinforced thermoplastic (FRTP) skins and low-density core of a thermoplastic material, thermoformed articles made therefrom, and more particularly an assembled container structure comprised of the thermoformed FRTP sandwich panels and attachment hardware.
2. Reference to Prior Art
Container structures such as those used for land, sea and air transport of goods having multi-piece metallic constructions are known. These structures make use of monocoque designs wherein relatively thin gage sheets forming a shell are mechanically fastened to angle, hat-section, doublers or similar stiffening elements. The monocoque shell structure formed is thus load bearing through the stiffening elements. Such structures are heavy due to their basic metallic construction, the use of robust stiffening elements, and the presence of mechanical fasteners required to assemble the shell and stiffeners. As the stiffening elements of a monocoque design are typically located on the interior of the shell structure, the stiffening elements of the design limit the useful volume of the container and interfere with internal loading of such containers as do the mechanical fasteners which protrude into the container volume.
These metallic structures are also susceptible to short lifecycles due to physical damage from mis-handling and their inherent lack of damage tolerance. Additionally, corrosion damage from their exposure to a harsh environment including fluctuating temperature extremes, water, ice, oils, solvents, and salt shortens their useful life cycle. Often an entire container is replaced where only stiffening elements or shell elements are damaged rather than performing a limited repair on the damaged element due to the load-bearing capacity of the individual elements.
Fiber Reinforced Plastics (FRP) are a non-metallic, composite material of a first, reinforcing element such as fiberglass, carbon, aramid fiber or woven form thereof which is encapsulated and bound within a second, matrix element such as a cured or hardened plastic of polyester, epoxy or other resin. Structures made from FRP's benefit from the composite synergy of the two, or more, constituent elements namely higher specific strength to weight ratios of FRP over conventional metallic structures such as aluminum or steel and are thus lighter in weight. Generally, when compared to their conventional metal counterparts, FRP's show better corrosion resistance, improved impact and damage tolerance, and lower piece/part count due to the increased complexity of designs possible with FRP's. For these and other benefits, FRP's have been integrated into aerospace, automotive, recreational, and industrial applications as direct replacement for metal structures. One such example is the use of polyester-fiberglass FRP in the marine industry for ship hulls, bulkheads, and decks. A second example is the use of carbon-epoxy FRP in aerospace applications such as aircraft fuselage and flight control structures.
A first type of FRP materials incorporates a single or multiple layer of FRP material consolidated or pressed into a sheet or panel often referred to as a laminate. While exhibiting increased strength to weight performance over metallic structures, replacing metallic structures with FRP laminates has met with mixed results. The nature of their multi-constituent fiber and matrix-binder form invites separation of the constituents at the interface of fiber and matrix-binder under concentrated, high stress conditions, particularly at attachment points and impact damage from handling or adverse environment conditions. Although generally more damage tolerant than metallic structures, FRP structures do not have well defined, time-proven means of repairing local damage to insure structural integrity of the whole. The FRP laminate construction often incorporates monocoque design utilizing shell, stiffening elements, and fasteners. Thus weight and damage repair/replace issues minimize the benefits of 100% FRP.
A second type of FRP materials incorporates a sandwich construction wherein a low density core material of foam or cellular construction is sealed at its surfaces by thin layers of FRP laminate material or skins. One such core material is honeycomb, a nodal arrangement of thin walled, parallel cells comprised of aluminum, coated paper, polymeric or other material. Sandwich structure FRP's exhibit superior stiffness and high strength to weight ratios compared even to solid panels of FRP. However, like FRP laminate materials, repair of localized damage to sandwich structure to insure structural integrity of the whole is more art than science. Also, honeycomb core of aluminum or coated paper is susceptible to moisture ingression, which causes corrosion, weight increase and/or sacrifices structural integrity and performance.
The low density foam or honeycomb core also presents serious issues in mechanically attaching the FRP sandwich panel to another structure. Such core materials do not resist bearing or pull-out load well and fail under such conditions if un-reinforced. Thus, local reinforcement of the attachment area or special fastener inserts adding weight, special manufacturing steps and interposing dissimilar materials is necessitated. The dissimilar materials raises CTE and bond integrity issues of concern to the structure. Hence, manufacturing the FRP sandwich structure often requires design specific sculpting or forming of the core in consideration of panel edge core-crush as well as appropriate configuration for mechanical attachment to the sandwich panel.
The integration of FRP laminate panels and FRP sandwich panels into applications where metallic structures are replaced has met with mixed success. While light weight, stiff structures with reduced part count can be achieved, these FRP structures have their own shortcomings including: limited design configurations and reduced weight savings, particularly on (mechanical) attachment to other components; design specific, low rate manufacturing techniques for a given configuration and desired performance requirements; damage tolerance issues from stress, physical impact and environmental exposures. As with monocoque design metallic structures, the use of FRP panel and FRP sandwich panel materials in monocoque container structures often results in replacing an entire structure where only a component has been damaged or its structural integrity suspect.